Calcining apparatus and process of use

ABSTRACT

Disclosed is an apparatus for the calcination of materials using low temperature heating and indirect heating for calcination. Also disclosed are a variety of processes for calcination of materials which have-reduced emissions of pollutants compared to conventional processes.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/058,643, filed Sep. 11, 1997.

FIELD OF THE INVENTION

[0002] The present invention relates to an apparatus for the calcinationof materials and uses therefor.

BACKGROUND OF THE INVENTION

[0003] A variety of industrial processes involve the use of calcinationto thermally decompose materials either to aid in the purification ofmaterials or for use in an industrial process. Generally, calcinationprocesses involve exposing the materials to be calcined to heat tothermally decompose the materials. Thus, calcination differs fromthermal drying of materials in which free water is driven off byexposure to increased temperatures. In contrast, calcination involveschanging the chemical composition of the material.

[0004] A number of apparatus are known for calcination processes. Forexample, rotary direct-fired calciners use an open flame as a heatsource and therefore, necessitate the use of combustion air. Also,vertical fluid bed calciners use heated gas in direct contact with thematerial to be calcined.

[0005] Despite the well-known use of calcination, a number of problemsexist in the use of conventional calcination processes. For example, theemission of by-products such as particulates causes pollution concerns.Additionally, a number of calcination processes are not energy efficientbecause much of the energy from the process is released to theatmosphere in the form of heat.

[0006] Further, many calcination processes which operate at hightemperatures, such as use of open flame calciners, unevenly heat thematerial to be calcined. For example, in open flame rotary calciners,material contacting the flame may experience a temperature close to1000° C., even though the average temperature in the calciner may besignificantly below that temperature. In this manner, some particles maynot be fully calcined and some may be combusted. Alternatively, somelarger particles may be calcined on the outside, but not on the insideof the particle. This type of disadvantage can also have significantnegative effects on downstream processing because the material exitingthe calcination process is not uniform in its chemical composition.Therefore, subsequent processing will have more variable results than ifthe material from the calcination process was uniform in nature.

[0007] As a result of the above disadvantages of known calcinationtechnology, there remains a need for improved calcination apparatus andmethods of use.

SUMMARY OF THE INVENTION

[0008] One embodiment of the present invention is an indirect heatcalcination apparatus for calcining materials. The apparatus includes afeed inlet, a calcining chamber which is interconnected to the feedinlet, an indirect heating element within the calcining chamber totransfer heat from a heated fluid to the material, a bed plate locatedbelow the indirect heating element within the calcining chamber, and aproduct collection chute which is connected to the calcining chamber.The apparatus can also include a plurality of holes on the bed plate anda gas inlet for introducing a fluidizing gas into the apparatus throughthe bed plate holes. The apparatus can include an exhaust port locatedabove the calcining chamber. The exhaust port can also include anexpansion chamber for slowing the velocity of gas exiting the calciningapparatus. The indirect heating element of the apparatus can be, forexample, coils within the calcining chamber which conduct the heatedfluid through the chamber. Thus, the indirect heating element caninclude a fluid inlet port and a fluid outlet port. The apparatus canalso include a plurality of calcining zones which are defined bycompartmental walls.

[0009] The present invention includes a calcining process for treating asaline mineral which includes introducing the saline mineral to acalcining chamber, heating the saline mineral to a temperature of lessthan about 350° by contacting it with an indirect heating element andremoving the calcined material from the chamber. In this embodiment, thecalcining chamber can include a bed plate located below the indirectheating element and having a plurality of bed plate holes and a gasinlet for introducing a fluidizing gas into the chamber through the bedplate holes. The calcining apparatus can also include an exhaust portlocated above the calcining chamber which can have an expansion chamberfor slowing the velocity of exiting gas. The apparatus can also includea plurality of calcining zones defined by compartmental walls.

[0010] Other processes of the present invention include processes forcalcining material and subsequent processing of the material. Forexample, such processing can include purification, such ascrystallization.

[0011] An additional process of the present invention is a method forreducing the emission of pollutants during calcining. This processincludes heating a saline mineral in a calcining vessel wherein thecalcination step produces a gas comprising water vapor and a pollutant.The calcining gas is removed from the calcining vessel to an outlet andat least a portion of the water vapor in the calcining gas is condensed.In this manner, a portion of the pollutants in the calcining gas areremoved. This process can also include the use of a heat source forcalcining which is not in direct fluid communication with the materialto be calcined. In further a aspect, the material is calcined attemperatures less than about 250° C.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a graph illustrating the settling characteristics oftrona calcined in a CO₂ atmosphere at various temperatures;

[0013]FIG. 2 is a graph illustrating the settling characteristics oftrona calcined in an air atmosphere at various temperatures;

[0014]FIG. 3 a graph illustrating the settling characteristics of tronacalcined in a CO₂ atmosphere at various temperatures and treated toremove magnetic impurities; and

[0015]FIG. 4 is a table providing data for various settling andcompositional characteristics of trona calcined in a variety oftemperatures and atmospheres.

[0016]FIG. 5 is a side view of an indirect heat calcining apparatus ofthe present invention.

[0017]FIG. 5A is an illustration of a one-piece exhaust port containingan expansion chamber.

[0018]FIG. 6 is a plan view of an indirect heat calcining apparatus ofthe present invention.

[0019]FIG. 7A is an illustration of a bed plate having a fluidizing gasinlet holes.

[0020]FIG. 7B is an illustration of bed plate having a fluidizing gasinlet holes and a gas-flow deflector

DETAILED DESCRIPTION OF THE INVENTION

[0021] In various embodiments of the present invention, processes andapparatus involve the use of a calcining step with low temperatureheating of the feedstream at temperatures lower than conventionalcalcination, such as in direct fired rotary kiln calciners. Moreparticularly, the calcining step of the present invention includesheating a feedstream to a temperature of less than about 350° C., morepreferably less than about 250° C., and more preferably at a temperaturefrom about 120° C. to about 250° C. As used herein, reference to heatinga feedstream to less than a certain temperature refers to raising thetemperature of the particles in the feedstream within the statedtemperature constraints, and not to the temperature of the ambientatmosphere in the-calciner or to the temperature of the heat transfermedium. Moreover, reference to heating a feedstream to less than acertain temperature requires that no substantial portion of particles inthe feedstream be heated in excess of the stated temperatureconstraints. Thus, it should be recognized that while substantially theentire feedstream is maintained within the temperature constraints,particles which actually come into contact with a heat transfer surface,such as a heated tube, may exceed the temperature constraints. Moreparticularly, however, no more than about 15 wt. % of the feedstreamshould be heated in excess of the stated temperature constraints, morepreferably no more than about 10 wt. %, and most preferably no more thanabout 5 wt %. In another aspect, no portion of the material in thefeedstream is heated in excess of about 450 C.

[0022] Calcination in accordance with these temperature constraints ofthe present invention provides a number of previously unrecognizedsignificant benefits. As discussed in more detail below, the amount ofpollutants from the calcination process is reduced with low temperaturecalcination. For example, low temperature calcination does notvolatilize as many organic compounds from insoluble impurities as midand high temperature calcination. Thus, fewer volatile organic compound(VOC) pollutants are generated by calcination. Also, fewer solubleorganic compounds, such as sulfonates, are generated. Additionally,benefits in subsequent processing are obtained.

[0023] In a preferred embodiment, the calcination temperatureconstraints are more readily achieved by controlling the particle sizeand particle size distribution of particles in the feedstream. By havinga relatively small particle size with a relatively narrow particledistribution, particles in the feedstream can be evenly heated to meetthe temperature constraints as discussed above. More particularly, thefeedstream to the calciner is typically comminuted to reduce theparticle size. For example, the feedstream can be comminuted to aparticle size of less than about ¼ inch, alternatively, less than about6 mesh, and alternatively, less than about 20 mesh. In addition, thefeedstream is preferably sized into multiple size fractions forcalcining. More particularly, the feedstream is sized into 3 or moresize fractions, more preferably 5 or more size fractions, and mostpreferably 7 or more size fractions. In this manner, it is more likelythat sufficient heating of all the particles will occur to completelycalcine them without excessively heating smaller particles in excess ofthe temperature constraints identified above. Thus, in a furtherembodiment, the present process includes calcination of at least about95 wt. % of the feedstream, more preferably, at least about 98 wt. %,and most preferably, at least about 99.5 wt. %.

[0024] In a further embodiment of the present invention, the step ofcalcining is conducted by heating a feedstream in an inert atmosphere toproduce a calcined material. The term inert atmosphere refers to anyatmosphere which is less oxidizing than air. For example, an inertatmosphere can be an atmosphere of carbon dioxide. Alternatively, thecarbon dioxide can also include water vapor and/or air. As discussed inmore detail below, some gaseous by-products of calcination, such ascarbon dioxide and water vapor generated as part of a calcining gas insome processes can be recycled and used as a fluidizing gas in afluidized bed calciner.

[0025] The calcining step is preferably performed utilizing an indirectheating process in a calcining vessel such as a fluidized bed reactor.In the indirect heating process, the combustion gases from the heatsource are not in direct fluid communication with the material beingcalcined, but rather provide heat to the material by conduction through,for example, heating coils, as described in more detail below.

[0026] The step of indirectly heating material for calcining comprisesthe steps of heating a fluid and bringing the heated fluid into thermalcommunication with material in the calcining vessel. As used in thisinvention, a “fluid” refers to a gas or a liquid medium. This step canbe accomplished utilizing a heat source which provides the heated fluidto coils positioned within the interior of the calcining vessel. In oneembodiment, the heat source is a steam boiler and the fluid is steam.Alternatively, the fluid may comprise oil or any other appropriatemedium. The step of heating the fluid can comprise the steps ofcombusting an energy source to produce heat and combustion gas,transferring at least a portion of the heat to the fluid, and directingat least a portion of the combustion gas through a combustion gas outletwhich is not in direct fluid communication with the calcining vessel.

[0027] Referring to FIG. 5 there is shown one embodiment of an indirectheat calcination apparatus of the present invention. The indirect heatcalcination apparatus 10 includes an indirect heating element 14 locatedwithin the calcining chamber 16 for providing indirect heat to thematerial. The indirect heating element 14 can be any conduit that allowsa fluid to flow within its walls while facilitating the transfer of heatfrom the fluid to the material. As described above, the fluid is heatedto a desired temperature and enters the indirect heating element 14 atan inlet port 18 and travels through the entire length of the indirectheating element 14 and exits through the outlet port 22. As the fluidtravels through the indirect heating element 14, heat is transferredfrom the fluid to the indirect heating element 14 and ultimately to thematerial to be calcined. In this manner, the material is calcinedwithout being exposed to a direct flame or heating fluid. Typically, asufficient amount of material is added to the indirect heat calcinationapparatus 10 to cover the entire indirect heating element 14 within thecalcining chamber 16. However, a smaller amount of the material can becalcined using the apparatus of the present invention.

[0028] In order for an efficient heat transfer to occur, it is preferredthat the indirect heating element 14 be made from a material which is agood heat conductor. Preferably, the material of indirect heatingelement 14 is selected from the group consisting of a metal such ascopper, steel, iron, nickel, zinc, stainless steel and mixtures thereof;ceramics; and composites. More preferably, the material of indirectheating element 14 is selected from the group consisting of steel andstainless steel and most preferably, is stainless steel.

[0029] As described above, the fluid for providing the heat forcalcination can be any liquid or gas which can be heated to asufficiently high temperature required for calcination. Such fluidsinclude water, steam, oil, and gases, including air. For calciningtrona, preferably the fluid is steam.

[0030] Again referring to FIG. 5, it is preferred that the fluid inletport 18 is located above the fluid outlet port 22. This arrangementensures that a lower amount of energy is required to operate theindirect heating element 14 because gravity aids in removing fluid fromthe indirect heating element 14. Moreover, when a gas such as steam isused, it is possible that some of it may condense to a liquid form asthe heat is transferred to the indirect heating element. The presence ofa condensed liquid within the indirect heating element 14 reduces theamount of heat transferred to the indirect heating element 14 becausesome of the energy will be used to heat the condensed liquid. In orderto reduce this problem, the inlet port 18 is located above the outletport 22 to facilitate the removal of any condensed liquid.

[0031] The indirect heating element 14 can be positioned within theapparatus 10 such that the indirect heating element 14 traverses backand forth across the apparatus and from top to bottom. This arrangementprovides a large indirect heating element surface area. However, evenwith this arrangement, the amount of surface area of the indirectheating element 14 is limited; therefore, not all of the materialparticles will come in direct contact with the indirect heating element14 when the material is stationary within the apparatus. Although all ofthe particles can be heated to a desired calcination temperature byprolonged exposure to the indirect heating element 14-and allowing theheat to transfer from one particle to another and eventually reaching anequilibrium, this method of stationary indirect heat calcinationrequires a large amount of energy and time rendering the apparatusrather inefficient. To expedite the calcination process and/or to reducethe amount of energy required, substantially all of the particles withinthe calcining chamber 16 can be made to be dynamic, i.e., non-stationarywithin the calcining chamber 16, during at least a portion of thecalcination process. Any method of creating a dynamic motion of theparticles can be used such as stirring, shaking and agitating.

[0032] In one particular embodiment, the particles are placed on top ofthe bed plate 26 and are fluidized by a fluidizing gas which isintroduced into the calcining chamber through a plurality of bed plateholes 30. This fluidization process causes a juggling effect of theparticles and allows more particles to come in a direct contact with theindirect heating element 14, resulting in a relatively even distributionof heat among the material particles. In addition, this fluidizationprocess can be used to separate the particles based on the difference indensity. The juggling effect provided by the fluidizing gas allowsrelatively heavy particles to settle to the bottom of the pile whileallowing relatively light particles to “float”, i.e., concentrate, tothe top of the pile.

[0033] The primary heat transfer mechanism is the material to coilcontact and not the material to fluidizing gas contact. Therefore, thefluidization gas velocity and volume has to be low or kept to a minimumto maximize the contact of the material to the indirect heating coils.This concept is contrary to current technology where the coils in afluid bed calciner are used to heat the fluidizing gas which in turn isused to heat the material. In such a process, large volumes offluidizing gas is required for the heat transfer to the material to takeplace.

[0034] The indirect heat calcination apparatus of the present inventioncan also include a gas plenum 34. The gas plenum 34 may be locatedunderneath the bed plate 26 to provide a substantially equal gaspressure throughout the bed plate holes 30. These holes can be angled,vertical or perpendicular to the direction of gas from the plenum to thecalcination bed. Angled holes will aid in the direction of flow ofmaterial through the calcining zone. In this embodiment, the angle ofthe hole must be greater than the angle of repose of the material beingcalcined to prevent material from falling into the hole. Moreover, thepresence of a gas plenum 34 in the apparatus 10 also reduces apossibility of particles falling through the bed plate holes 30 andblocking the flow of fluidizing gas into the apparatus. In operation,the fluidizing gas is introduced into the apparatus 10 through a gasinlet 38 into the gas plenum 34. As some materials are calcined, thedensity of the material decreases. With the lower density, the amount offluidization gas needed decreases. Therefore, individual flow controlsfor the fluidizing gas to each calcining zone is preferred. In oneparticular embodiment, fluidizing gas is heated to about the sametemperature as the temperature of the heating coils to prevent coolingof material particles and/or to maintain the temperature above the dewpoint. In this manner, condensation of water in the fluidizing gas isavoided.

[0035] The indirect heat calcination apparatus 10 of the presentinvention can also include an exhaust port 42 to prevent excess pressurebuild-up within the apparatus or to remove any volatile compounds whichare released or generated from the material during the calcinationprocess. The exhaust port is located above the material level to allowthe fluidizing gas to fluidize the material particles. Since theparticles are not all identical size, it is expected that some of thelighter particles, e.g., smaller particles or the material dust, will becarried into the exhaust port 42.

[0036] In order to reduce the amount of particles removed from theapparatus by the action of the fluidizing gas, the indirect heatcalcination apparatus 10 of the present invention can also include anexpansion chamber 46 which is located below or near the exhaust port 42.The cross-sectional area of the expansion chamber 46 is larger than thecross-sectional area of the calcining chamber 16, and as a result thevelocity of gas, i.e., the flow rate, decreases as the fluidizing gasflows from the calcining chamber 16 into the expansion chamber 46. Thisdecrease in the fluidizing gas flow rate results in some of the solidscarried upward into the expansion chamber 46 by the fluidizing gas tosettle and drop back down into the calcining chamber 16, thus reducingthe particulate emission from the calcination apparatus. The expansionchamber is even more important in a case where the material releasesvapor upon during calcining. This is the case with trona, where carbondioxide and water vapor are released. This release of vapors increasesthe volume of fluidizing gas in the calciner bed and therefore increasesthe velocity of the fluidizing gas. This effect further entrainsparticles that can be returned to the calciner bed with the use of anexpansion zone. Moreover, the exhaust port 42 can be fitted with otherapparatus to collect any material that is released through the exhaustport 42. For example, a condenser can be fitted to the exhaust port 42to condense and collect water vapor or other useful materials, a filtercan be fitted to further reduce the amount of particulate matter that isreleased into the environment, or a gas collector can be fitted tocollect or recycle the fluidizing gas or other gases which may bereleased through the exhaust port 42. Alternatively, the exhaust port 42and the expansion chamber 46 can be a single unit piece, i.e, theexpansion chamber 46 can be an integral part of the exhaust port 42 asshown in FIG. 5A.

[0037] A process for indirect heat calcination of a material using theapparatus of the present invention will now be described in reference toFIG. 6 which illustrates the indirect heat calcination apparatus havingfour different calcining zones 54, 58, 62 and 66, that are separated bythree compartmental walls 70, 74 and 78. The material can be pretreated,e.g., comminuted, size separated and/or dried, prior to being calcinedusing the indirect heat calcining apparatus of the present invention. Ina typical operation, a feedstream of comminuted material is introducedinto the apparatus 10 through a feed inlet 50. In order to reduce theamount of agglomeration of particles due to the moisture that is presentin the calcining atmosphere, the indirect heat calcining apparatus 10can also include a predried-gas inlet (not shown) near the feed inlet 50for reducing the moisture level of the particles or moisture in theatmosphere from coming in contact with particles. Alternatively, thefirst calcining zone 54 can be used as both a pre-drying region as wellas the first calcining zone. However, if a separate predried-gas inletis used, the diameter of the predried-gas inlet is selected to ensurethat a sufficient gas flow rate is maintained to provide a sufficientlevel of pre-drying. Factors influencing the diameter of thepredrying-gas inlet include the particle size of the material, themoisture level of the calcining atmosphere, density of the material, andthe desired gas flow rate. Pre-heating with dry fluidization gas is usedto allow material of a lower temperature to enter the calciner and mixinto the material bed before condensation can occur on the enteringmaterial. Condensation on the material can cause agglomerates to form orcaking. The temperature of this gas is less than the calciningtemperature, but above the dew point for the material's moisturecontent. It is also important not to let the material reach atemperature above the calcination temperature before it is fluidized,otherwise moisture released during calcination can cause the particle tocake. To limit the temperature in the first calcining zone to preventcondensation and caking of material, the amount of heating element, suchas coils, for conducting heating fluid in the first calcining zone canbe less than in other zones. In addition, the first calcining zone canalso include already calcined material to reduce the amount of gasreleased from calcination in the first zone.

[0038] The primary purpose of heating the material with the predryinggas is to reduce the amount of moisture present where the ore enters thecalciner. Thus, although some of the material may be calcined duringthis pre-drying process, the majority of the material is not calcined bythis pre-drying process. Drying the material reduces the agglomeration,thus maintaining the high surface area of the particles which is desiredfor indirect heat calcination. Particles having a high surface area tovolume ratio can be calcined more quickly and/or more efficiently thanthe particles having a low surface area to volume ratio.

[0039] The rate of pre-dried gas flow depends on a variety of factorsincluding the feed rate, the particle size and the density of impuritiesand/or the material being calcined.

[0040] As the materials are introduced into the first calcining zone 54,they are fluidized by a fluidizing gas and are heated by the indirectheating element 14. The materials then flow to the second calcining zone58, the third calcining zone 62 and the fourth calcining zone 66, in asuccessive manner. Although FIG. 6 shows each calcining zones having itsown gas inlet 38 and flow control (not shown), the apparatus can haveless than one gas inlet per calcining zone for providing the fluidizinggas to the entire bed plate 26 of the apparatus 10. As more material isfed to the first zone 54, at least a portion of the materials in thefirst zone 54 flows in to the second zone 58 through the opening 82. Asthe height of the feedstream in the second zone 58 reaches the top ofthe compartment wall 74, the material overflows into the thirdcalcination zone 62. At least a portion of the material in the thirdcalcination zone 62 then flows into the fourth calcination zone 66through the opening 86. The calcined material then overflows into theproduct collection chute 90 where it is collected.

[0041] Having a multiple calcination zone provides a longer averageresidence time for material within the indirect heat calcinationapparatus 10. The desired average residence time depends on a variety offactors including the temperature of the indirect heating element 14,the feed rate, the particle size of the feed, the completeness ofcalcination of the feedstream and the amount of time required to calcinethe material. The underflow/overflow design forces contact of thematerial being calcined with the coils.

[0042] Referring again to FIG. 6, as the material flows into the secondcalcining zone 58, the fluidizing gas can be used to provide a densityseparation as described above. In this manner, the lighter materialswill be concentrated on the top portion and the denser materials will beconcentrated on the bottom portion. In the case of trona ore beingcalcined, this means that the lighter anhydrous sodium carbonate and/ortrona will be on the top portion and the heavier impurities such asshortite, shale and/or pyrite will be concentrated in the bottomportion. A similar density separation can be achieved in the fourthcalcining zone 66. By allowing a means for removing the bottom impurityconcentrated portion in the second and/or the fourth calcination zones,a substantially purified calcined material can be collected through theproduct collection chute 90.

[0043] Alternatively, all the flow of the material from one calciningzone to another calcining zone can be made to proceed over thecompartmental walls, thus eliminating a need for openings 82 and 86. Oneway this can be accomplished is by increasing the fluidizing gas flowdirectly adjacent to the compartment wall to the point where allmaterial including high density material is forced to overflow thecompartmental wall. In order to prevent back-flow of the materials, thesuccessive compartment walls can be lower in height than the previouscompartment wall. In this manner, each successive calcining zone willcontain less amount of heavier impurities.

[0044] As shown in FIG. 7A, the bed plate holes 30 can simply be anopening, in which case the direction of the gas flow is substantiallyperpendicular to the opening of the bed plate holes 30 or can bedetermined by the direction of gas-flow prior to entering the bed plateholes 30.

[0045] Alternatively, as shown in FIG. 7B, the bed plate holes 12 canalso include a gas flow deflector 94 which is placed above the bed plateholes 30. The deflector 94 can serve a multiple purposes. For example,it can be designed to prevent any particles from entering, or fallingthrough, the bed plate holes 30. Another way to achieve this result isto punch holes in the plate at an angle. In addition, the movement ofthe particles towards the product collection chute 90 can be facilitatedby using the gas flow deflector 94 to introduce the fluidizing gas inthe direction towards the product collection chute 90. Thus, the averageresidence time of the particles can be controlled by using thefluidizing gas and the gas flow deflector 94.

[0046] The materials calcined using the indirect heat calcinationapparatus of the present invention have unique product characteristicsbecause of the use of low heat and relatively even heating of theparticles during the calcination process. In addition, the materials canbe further processed, including dry separation such as densityseparation, electrostatic separation, magnetic separation, calorimetricseparation; and wet separation such as recrystallization methods andevaporative crystallization methods.

[0047] The utilization of indirect heating for calcining materialprovides significant benefits in that it significantly reduces theamount of gas flowing through the fluidized bed because no combustiongas flows through the bed. In this manner, a significantly lower amountof particulates from the material are entrained and need to be removedfrom exhaust gas from the calcining operation. More specifically, theamount of gas required for fluidization is typically about 80% less thanthe amount of gas produced during the combustion necessary to producesufficient heat for the calcining process (e.g., utilizing natural gasin a steam boiler). Accordingly, by utilizing a source of gas forfluidization which is different than the combustion gases, a smalleramount of fluidizing gas can be used. Further, the smaller amount meansthat the fluidizing gas will flow at lower velocities, therebypotentially reducing particulate entrainment even further. In addition,less fluidizing gas means that less gas needs to be scrubbed forparticulates before emission, thereby reducing the costs of thecalcining process.

[0048] It is well known that some calcining processes produce calcininggas having a significant amount of water vapor. For example, in theinstance of calcining trona to produce anhydrous sodium carbonate,calcining three moles of trona produces five moles of water and one moleof carbon dioxide. In order to reduce the amount of calcining gasexiting the system, the process may further comprise the step ofcondensing at least a portion of the water vapor from the calcining gasby, for example, cooling the calcining gas. Such condensation step willreduce the calcining gas volume by as much as ⅚ths, thereby reducing theamount of calcining gas which must be treated. In addition to reducingthe volume of gas exiting the system, the condensing step also has ascrubbing effect on the calcining gas by removing particulates from thecalcining gas. It is believed that the amount of particulates removed isproportional to the amount of gas removed (i.e., as much as ⅚ths or morein the case of trona). It is estimated that the particulate emissionfrom a process for calcining trona ore in a direct-fired rotary calcineris typically about 6 lbs/ton of feed. By practice of the presentprocess, including indirect calcination and condensing water fromcalcining gas, the particulate emissions from calcination of trona orecan be less than about 3 lbs/ton of feed, more preferably less thanabout 1.5 lbs/ton of feed and most preferably less than about 1 lbs/tonof feed.

[0049] In a preferred embodiment, the condensing step for condensingwater from gas produced during calcining comprises two stages. In thefirst stage, a small amount (e.g., less than about 5%) of the watervapor within the calcining gas is condensed to significantly reduce theparticulate content of the gas. The first stage can be performedutilizing a water-cooled condenser, such as a tubed condenser. In thesecond stage, as much as 80% of the water vapor is condensed. Because ofthe reduction in particulate content resulting from the first stage, thewater condensed from the second stage is essentially distilled watergrade. A third stage may be added to further scrub particulates from thegas. For example, a high-efficiency venturi scrubber or electrostaticprecipitator may be used.

[0050] The water which is removed during the condensing steps can beutilized for other processes. For example, the condensed water may becooled (e.g., using air coolers) and then recycled and used as thecooling medium to condense further water vapor from the calcining gas bybringing the cooled water into thermal communication with thepre-condensed calcining gas. Further, the condensed water could beutilized for processes in other areas of a facility which involve theuse of water. The condensed water may also be treated and utilized foralmost any other appropriate purpose, such as for general water usage inthe facility (e.g., for cleaning, drinking water, etc.).

[0051] Calcining gas which is produced during the calcining process maybe removed from the calcining vessel through a calcining gas outlet, andat least a portion of the calcining gas (proportional to the amount ofCO₂ produced in the calcining process) may be expelled through a stack.The expelled gas is preferably heated prior to exiting through the stackto inhibit condensation and plume formation at the stack outlet. Forexample, the expelled gas can be mixed with hot combustion gas fromheating fluid for indirect calcination.

[0052] Another portion of the calcining gas may be recycled back to theinlet of the calcining vessel and utilized for heating and fluidizingadditional material for calcining. Preferably, this gas is recycledand/or heated after the above-noted condensation step, thereby resultingin dry gas as the heating and fluidizing medium. The recycled gas may beheated (e.g., by steam coils) in order to bring the gas up to atemperature prior to entry into the calcining vessel. In one embodiment,the recycled gas temperature is between about 120° C. and about 200° C.,and is preferably about 140° C. This recycling of gas is beneficial inthat it utilizes latent heat within the calcining gas as part of theenergy required for calcining, rather than heating ambient temperaturegas up to calcining temperature. Further, such recycling reduces the gasrequirements and emissions of the process by eliminating the need forfresh gas.

[0053] In a further embodiment of the present invention, a densityseparation is conducted in the calcining vessel. As some materials arecalcined, they lose mass while impurities in the material do not. Inthis manner, the apparent density of the calcined material is lesscompared to the impurities and can, therefore, be separated on a densityseparation basis. For example, as trona, containing impurities such asshale, pyrite and/or shortite, is calcined, the sodium carbonateparticles lose mass and become less dense, thereby creating asignificant density difference between the anhydrous sodium carbonateand the impurities. Thus, in a fluidized bed, the anhydrous sodiumcarbonate will migrate to the top of the bed and the denser impuritieswill migrate to the bottom. For example, an average apparent density ofcalcined trona is less than about 1.6. An average density of a bottomimpurity stream in this embodiment is greater than about 2.1, morepreferably greater than 2.3, and most preferably greater than about 2.5.Thus, a further aspect of the present invention is to calcine trona andremove a particle stream comprising impurities from the bottom of thecalciner bed. As will be appreciated, depending on how much of animpurity stream is taken, the impurity stream may include some sodiumcarbonate. However, a bottom stream will contain primarily impurities,such as shale, pyrite and/or shortite, and the concentration of sodiumcarbonate in top stream is greater than in the bottom stream. Moreparticularly, the concentration of sodium carbonate in the top streamwill be at least about 96 wt. %, more preferably at least about 98 wt.%, and most preferably at least about 99 wt. %.

[0054] After calcination of materials, subsequent processing of somesort is typically conducted on the material. Often, such subsequentprocessing involves purification. In a preferred embodiment of thepresent invention, the material being calcined is a saline mineral, andthe calcined saline mineral is subsequently processed by purification ina crystallization process. In a further preferred embodiment, the salinemineral is trona. By way of example, a particular crystallizationprocess for purification of saline minerals will be described in detail.Use of the present calcination process and apparatus (specifically, lowtemperature calcination) provides significant benefits in terms ofcrystallization processes for saline minerals, including among otherthings, larger crystals.

[0055] As used herein, the term “saline mineral” refers generally to anymineral which occurs in evaporite deposits. Saline minerals that can bebeneficiated by the present process include, without limitation, trona,borates, potash, sulfates, nitrates, sodium chloride, and preferably,trona.

[0056] The purity of saline minerals within an ore depends on thedeposit location, as well as on the area mined at a particular deposit.In addition, the mining technique used can significantly affect thepurity of the saline minerals. For example, by selective mining, higherpurities of trona ore can be achieved. Deposits of trona ore are locatedat several locations throughout the world, including Wyoming (GreenRiver Formation), California (Searles Lake), Egypt, Kenya, Venezuela,Botswana, Tibet and Turkey (Beypazari Basin). For example, a sample oftrona ore from Searles Lake has been found to have between about 50% andabout 90% by weight (wt. %) trona and a sample taken from the GreenRiver Formation in Wyoming has been found to have between about 80 andabout 90 wt. % trona. The remaining 10 to 20 wt. % of the ore in theGreen River Formation sample comprised impurities including shortite(1-5 wt. %) and halite, and the bulk of the remainder comprises shaleconsisting predominantly of dolomite, clay, quartz, kerogen and iron,and traces of other impurities. Other samples of trona ore can includedifferent percentages of trona and impurities, as well as include otherimpurities. The present process can also be used with feedstreams havinglower impurity contents, including impurity levels as low as 0.1% byweight.

[0057] The crystallization process described herein is particularly welladapted for use with feedstreams having high contents of insolubleimpurities. For example, the present invention is suitable for use withfeedstreams having greater than about 4% by weight insoluble impurities,more particularly greater than about 15% by weight insoluble impurities,and even more particularly greater than about 30% by weight insolubleimpurities. The present process can also be used with feedstreams havinglower impurity contents, including impurity levels as low as 0.1% byweight.

[0058] The sodium carbonate resulting from calcination of trona, asdescribed above, is treated by purification in a crystallization processto remove insoluble impurities. A first crystallization process includescontacting the calcined feedstream comprising sodium carbonate andinsoluble impurities with a saturated sodium carbonate brine solution,the saturated sodium carbonate brine solution being maintained at atemperature between about 35° C. and about 112° C., more preferablybetween about 85° C. and about 112° C., and most preferably betweenabout 95° C. and about 112° C., to form sodium carbonate monohydratecrystals and separating the sodium carbonate monohydrate crystals fromthe saturated sodium carbonate brine solution, preferably on a sizeseparation basis. The sodium carbonate monohydrate crystals which areremoved from the brine solution can be dewatered, dried and eventuallyconverted to anhydrous sodium carbonate. Such a process is describedgenerally in U.S. Pat. No. 3,948,744 to Frint, which is herebyincorporated by reference.

[0059] In particular, sodium carbonate monohydrate crystals having acrystal size of greater than about 150 mesh, more preferably greaterthan about 100 mesh, and more preferably greater than about 80 mesh, canbe obtained by the present process. By forming large sodium carbonatemonohydrate crystals, significant advantages are obtained. For example,the ability to recover purified crystals on a size separation basis isenhanced. Larger crystals enable greater recovery yields when separatingcrystals from smaller insoluble impurities, such as in the case ofrecovering sodium carbonate from a feedstream of trona ore. Thus, in afurther aspect of the invention, the crystallization process isconducted in the absence of procedures, such as grinding or shearing,which significantly reduce crystal size in the crystallizationoperation.

[0060] As noted, a preferred method of recovery of sodium carbonatecrystals is on a size separation basis. Such a basis involves theseparation of sodium carbonate monohydrate crystals from impuritiesbased on differences in size between the sodium carbonate monohydratecrystals and the impurities. Typically, impurities which can occur inthe trona feedstream include iron-bearing materials, dolomite, shale,shortite, searlesite and northupite. It will be recognized that the sizeof insoluble impurities will not be affected by the recrystallizationprocess. Thus, the initial particle size of an insoluble impurity willbe the minimum particle size at which size separation of crystals canoccur. Moreover, the particle size of insoluble impurities can bereduced prior to introduction into the brine solution by grinding thefeedstream to smaller sizes. Typically, the feedstream has a particlesize of minus 100 mesh and more preferably minus 200 mesh. The sizeseparation is typically conducted at a size from about 80 mesh to about150 mesh, and even more particularly at about 100 mesh.

[0061] A significant advantage of the low temperature calcinationprocess described above is that subsequent recovery of impurities ismade easier. It has been determined that low temperature calcinationmakes the insoluble impurities less likely to break down into ultrafineparticle sizes, such as less than about 500 mesh. Thus, ease ofsubsequent recovery and denaturing of the particles is significantlyincreased.

[0062] Size separation can be affected by any known appropriate method.For example, screening or elutriation can be used. In the instance ofscreening, the oversize material from a first screening may betransferred to a repulping operation for suspension of crystals in theoversize fraction by adding clean liquor to a repulp tank to obtain amore efficient screening in a second size separation.

[0063] Upon introduction of a feedstream into a saturated brinesolution, a problem which can be encountered is clumping and poordispersion of sodium carbonate. In another embodiment of the invention,in order to avoid clumping and to allow for adequate dispersion of thesodium carbonate within the brine solution, the brine solution isagitated during introduction of the feedstream containing sodiumcarbonate. In another embodiment, the feedstream may be preheated to atemperature above about 175° C. and blown into the brine solution Oncesodium carbonate monohydrate crystals are separated from the saturatedbrine solution, the crystals are dewatered, such as by centrifugation.The crystals can then be converted to the anhydrous form of sodiumcarbonate after dewatering for use in industry, such as in theproduction of glass. Conversion of sodium carbonate monohydrate to theanhydrous form after dewatering provides significant advantages overconversion while in a slurry. To convert to the anhydrous form while ina slurry, the temperature of the slurry must be above the boiling pointof water. Thus, the process needs to be conducted in a pressurizedsystem. The equipment necessary for such systems introduces significantcost and complexity compared to the present process.

[0064] The size of the monohydrate crystals may be effected by varyingthe feed rate and/or temperature of the anhydrous sodium carbonateintroduced to the saturated sodium carbonate brine solution and byvarying the crystal size distribution of the sodium carbonatemonohydrate seed. Furthermore, appropriate residence times of sodiumcarbonate monohydrate crystals in the brine solution for crystallizationcan be selected by those skilled in the art. It should be recognized,however, that longer residence times will result in larger monohydratecrystals which can have significant advantages with respect to recovery,as discussed above. It is believed that residence times of the sodiumcarbonate monohydrate crystals in the brine solution could be as littleas fifteen minutes, but can be significantly longer as well. In onepreferred embodiment, the residence time of the crystallization can begreater than about one and a half hours, more preferably greater thanabout three hours and more preferably greater than about five hours. Itwill be recognized that residence time corresponds to feed rate into thecrystallizer. In a further embodiment, the feed rate into thecrystallizer is less than about 0.4 lbs. of anhydrous sodium carbonateper minute per gallon, more preferably less than about 0.3 lbs. perminute per gallon and even more preferably, less than about 0.2 lbs. perminute per gallon.

[0065] For example, by maintaining a crystal size distribution with ahigh degree of uniformity of size, crystals can be efficiently grown toa large size. That is, if crystal size distribution is widely spreadover a great number of small to large crystals, while some new crystalgrowth will be efficiently spent on making large crystals larger, somesuch growth will be inefficiently spent on making small crystals grow toa size that will still not be recovered because it will be below thesize separation cutoff Thus, a further aspect of the presentcrystallization process is to maintain a narrow crystal sizedistribution of sodium carbonate monohydrate seed crystals. This aspectof the invention is particularly important when the feedstream includesinsoluble impurities because adequate crystal growth is necessary toobtain crystals having a larger size than the insoluble impurities. Thisaspect of the invention can be accomplished by a variety of techniques.For example, by removing small crystals, either continuously orintermittently, from the crystallization vessel, the crystal sizedistribution will be narrowed with the average crystal size of theremaining crystals being greater than before removal. More specifically,crystals having a crystal size less than about 150 mesh, moreparticularly less than about 200 mesh and even more particularly lessthan about 400 mesh can be removed for this purpose.

[0066] In a further embodiment of the present invention, after recoveryof sodium carbonate from the saturated brine solution, sodium carbonatein the non-recovered portion can also be kept in the system forsubsequent recovery. As will be appreciated, the non-recovered portioncomprises insoluble impurities and residual sodium carbonate monohydratecrystals having a particles size smaller than the size separationcutoff. For example, when recovery is made on a size separation basis,the non-recovered portion will have crystals with a size below the sizeseparation cutoff. In this instance, the non-recovered portion can betreated to recover sodium carbonate values from crystals which aresmaller than the size separation cutoff. Such a treatment can includedissolving the small crystals, such as by the addition of wash water,and then making a solid/liquid separation to remove solid impuritiesfrom the dissolved crystals. Then, the solution can be recycled to otherpoints in the process for use in washing, etc., so that the solutionultimately returns to the crystallization unit for recovery of dissolvedsodium carbonate. Alternatively, the water in the sodium carbonatesolution could be driven off (e.g., by heating) to recover the sodiumcarbonate by crystallization.

[0067] When insoluble impurities are removed by a solid/liquidseparation, most typically, the waste stream is sent to a clarifier,settling tank or other gravity purification apparatus. As illustratedbelow in the Example section, calcination in accordance with the presentinvention and in particular, in accordance with temperature constraintsresults in faster and more compact settling of insoluble impurities.This result provides significant cost and operational advantages in theprocess. Because the settling of impurities occurs more quickly andthus, is more efficient, the capital requirements for a plant using thisprocess are significantly lower. In addition, the resulting muds have ahigher solids content and therefore, can be readily disposed of. Moreparticularly, insoluble impurities produced by the process of thepresent invention, in the absence of a flocculant or other settling aid,can settle to a final density of at least about 20% solids, morepreferably to a final density of at least about 25% solids, and mostpreferably to a final density of at least about 30% solids.

[0068] A second crystallization process includes dissolving theanhydrous sodium carbonate in solution to form a sodium carbonatesolution. At least a portion of insoluble impurities present in thecalcined sodium carbonate are separated from the sodium carbonatesolution. Sodium carbonate monohydrate crystals are then formed from thesodium carbonate solution. This process is generally discussed in U.S.Pat. No. 3,644,331 to Seglin et al., which is incorporated herein byreference. The sodium carbonate monohydrate crystals which are producedcan be dewatered, and eventually converted to anhydrous sodium carbonateby drying or calcining.

[0069] Embodiments of the present invention can be conducted incombination with other processes known for treating saline minerals andin particular, trona ore. For example, such other processes aregenerally described in the published Patent Cooperation Treatyapplications PCT/US96/00700 for METHOD FOR PURIFICATION OF SALINEMINERALS and in PCT/US94/05918 for BENEFICTATION OF SALINE MINERALS, thedisclosures of which are incorporated herein by reference in theirentirety. More particularly, separation steps, such as magneticseparation, electrostatic separation, and density separation, can beconducted in conjunction with processes as described herein in detail.Similarly, separation steps based on other properties can be used aswell. For example, for ores or treated ores in which fractions havingdifferent colors or sizes corresponding to differences in purity,separations can be made on the basis of such properties, as well.

[0070] The following experimental results are provided for purposes ofillustration and are not intended to limit the scope of the invention.

EXAMPLE

[0071] The following example illustrates the effect of calciningtemperature and atmosphere on the settling of insoluble impurities introna ore.

[0072] Trona ore having a particle size of less than 20 mesh wascalcined at 150° C., 300° C., 450° C., or 600° C. in an atmosphere ofeither CO₂ or air. The ore was then ground to minus 100 mesh, and waterwas added to completely dissolve all soluble components (i.e., sodiumcarbonate) of the material. Some of the samples were first treated bymagnetic separation before the settling test. The samples were thenallowed to settle in graduated cylinders. The volume of settled solidmaterials was recorded over time to evaluate settling characteristics ofthe samples. Observations were made regarding the color of the liquid.The final solids content of the various samples, and the sodiumcarbonate content of the thickened pulp were determined. The results ofthe various settling tests are illustrated in the graphs in FIGS. 1-3and the chart in FIG. 4.

[0073] The foregoing description of the present invention has beenpresented for purposes of illustration and description. Furthermore, thedescription is not intended to limit the invention to the form disclosedherein. Consequently, variations and modifications commensurate with theabove teachings, and the skill or knowledge of the relevant art, arewithin the scope of the present invention. The embodiment describedhereinabove is further intended to explain the best mode known forpracticing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with variousmodifications required by the particular applications or uses of thepresent invention. It is intended that the appended claims be construedto include alternative embodiments to the extent permitted by the priorart.

What is claimed is:
 1. An indirect heat calcination apparatus forcalcining a material comprising: a) a feed inlet; b) a calcining chamberinterconnected to said feed inlet; c) an indirect heating element withinsaid calcining chamber for transferring heat from a heated fluid to saidmaterial; d) a bed plate located below said indirect heating elementwithin said calcining chamber; and e) a product collection chuteinterconnected to said calcining chamber.
 2. The apparatus of claim 1,further comprising: f) a plurality of bed plate holes on said bed plate;and g) a gas inlet for introducing a fluidizing gas into said apparatusthrough said bed plate holes.
 3. The apparatus of claim 2, wherein saidbed plate holes are selected from the group consisting of angled bedplate holes and bed plate holes comprising deflector plates.
 4. Theapparatus of claim 1, further comprising a gas plenum.
 5. The apparatusof claim 1, further comprising an exhaust port located above saidcalcining chamber.
 6. The apparatus of claim 1, further comprising anexpansion chamber interconnected to said exhaust port and said calciningchamber.
 7. The apparatus of claim 1, wherein said indirect heatingelement further comprises a fluid inlet port and a fluid outlet port. 8.The apparatus of claim 1, wherein said apparatus comprises a pluralityof calcining zones, and wherein said calcining zones are defined bycompartmental walls.
 9. The apparatus of claim 8, wherein each of saidcalcining zones comprises means for controlling the flow of fluidizinggas and each of said means is separately controlled.
 10. The apparatusof claim 8 further comprising an interconnecting opening near the bottomof alternating compartmental walls.
 11. The apparatus of claim 10,wherein compartmental walls without interconnecting openings are shorterthan walls with interconnecting openings.
 12. The apparatus of claim 1,further comprising a means for fluidizing said material with a flow offluidization gas.
 13. The apparatus of claim 12, further comprisingmeans for increasing the flow of fluidization gas to selectivelyfluidize dense material.
 14. The apparatus of claim 1, furthercomprising a means for removing at least a portion of impurities on adensity separation basis.
 15. A process for treating a saline mineral,wherein said saline mineral comprises insoluble impurities, comprisingthe steps of: (a) introducing a feedstream comprising said salinemineral and insoluble impurities through a feed inlet to a calciningchamber; (b) heating said saline mineral to a temperature of less thanabout 350° C. by contacting said saline mineral with an indirect heatingelement to calcine said saline mineral; (c) removing said calcinedsaline mineral from said calcining chamber through a product collectionchute.
 16. The process of claim 15, wherein said calcining chamberfurther comprises (i) a bed plate located below said indirect heatingelement and said bed plate comprises a plurality of bed plate holes and(ii) a gas inlet for introducing a fluidizing gas into said calciningchamber through said bed plate holes.
 17. The process of claim 15,wherein said calcining chamber further comprises a gas plenum.
 18. Theprocess of claim 15, wherein said calcining chamber further comprises anexhaust port located above said calcining chamber.
 19. The process ofclaim 15, wherein said calcining chamber further comprises an expansionchamber interconnected to said exhaust port and said calcining chamber.20. The process of claim 15, wherein said indirect heating elementfurther comprises a fluid inlet port and a fluid outlet port.
 21. Theprocess of claim 15, wherein said calcining chamber comprises aplurality of calcining zones, and wherein said calcining zones aredefined by compartmental walls.
 22. The process of claim 21, whereinsaid calcining zones further comprise interconnecting openings near thebottom of said alternating compartmental walls.
 23. The process of claim15, wherein said calcining chamber further comprises means forfluidizing said material.
 24. The process of claim 15, wherein saidcalcining chamber further comprises means for removing at least aportion of impurities on a density separation basis.
 25. A process forproducing sodium carbonate from a feedstream containing trona andinsoluble impurities, comprising the steps of: (a) heating saidfeedstream in a calcining apparatus to a temperature of less than about350° C. to form anhydrous sodium carbonate; (b) contacting saidanhydrous sodium carbonate with a saturated sodium carbonate brinesolution to form sodium carbonate monohydrate crystals; and (c)separating at least a portion of said sodium carbonate monohydratecrystals from at least a portion of said insoluble impurities to form animpurity stream.
 26. The process of claim 25, wherein said temperatureof heating is from about 120° C. to about 250° C.
 27. The process ofclaim 25, wherein a heat source in said heating step is not in directfluid communication with said feedstream.
 28. The process of claim 25,wherein said calcining apparatus is a fluidized bed reactor.
 29. Theprocess of claim 25, further comprising the step of comminuting saidfeedstream to provide a comminuted feedstream before step (a).
 30. Theprocess of claim 29, wherein particles in said comminuted feedstreamhave a particle size of less than about ¼ inch.
 31. The process of claim29, wherein said feedstream is sized into 3 or more size fractions. 32.The process of claim 25, wherein said heating step comprises the stepsof: heating a fluid; and bringing the heated fluid into thermalcommunication with said feedstream.
 33. The process of claim 32, whereinsaid step of heating said fluid comprises the steps of: (i) combustingan energy source to produce heat and combustion gas; (ii) transferringat least a portion of the heat to the fluid; and (iii) directing atleast a portion of the combustion gas through a combustion gas outletwhich is not in direct fluid communication with said calcining vessel.34. The process of claim 33, wherein said step of heating saidfeedstream further comprises the steps of: removing calcining gas fromsaid heating step through a calcining gas outlet; and combining at leasta portion of said calcining gas with at least a portion of saidcombustion gas.
 35. The process of claim 34, further comprising thesteps of removing said calcining gas from said heating step andcondensing at least a portion of water vapor from said calcining gas.36. The process of claim 35, wherein particulates are removed from thesaid calcining gas during said condensing step.
 37. The process of claim35, wherein said step of condensing at least a portion of said watervapor comprises the step of condensing said portion of water vapor bycooling said calcining gas.
 38. The process of claim 25, furthercomprising the step of separating a portion of said impurities from saidtrona before step (a) by a process selected from the group consisting ofmagnetic separation, electrostatic separation, density separation,calorimetric separation and size purification.
 39. The process of claim25, wherein the temperature of said saturated sodium carbonate brinesolution is from about 35° C. to about 112° C.
 40. The process of claim25, wherein the temperature of said saturated sodium carbonate brinesolution is at least about 95° C.
 41. The process of claim 25, whereinsaid separation of said sodium carbonate monohydrate crystals from saidsaturated sodium carbonate brine solution is by size separation.
 42. Theprocess of claim 41, wherein said sodium carbonate monohydrate crystalsseparated from said saturated sodium carbonate brine solution have aparticle size of at least about 100 mesh.
 43. The process of claim 41,wherein a non-recovered portion from said size separation step comprisesinsoluble impurities and said sodium carbonate monohydrate crystalshaving a particle size of less than about 100 mesh.
 44. The process ofclaim 43, further comprising the step of dissolving said sodiumcarbonate monohydrate crystals from said non-recovered portion andseparating said insoluble impurities from said dissolved crystals. 45.The process of claim 44, further comprising the step of recycling saiddissolved sodium carbonate monohydrate crystals from said non-recoveredportion by introducing a stream containing said dissolved sodiumcarbonate monohydrate crystals from said non-recovered portion into saidsaturated sodium carbonate brine solution.
 46. The process of claim 41,further comprising the step of drying or calcining said separated sodiumcarbonate monohydrate crystals and converting said separated sodiumcarbonate monohydrate crystals to anhydrous sodium carbonate crystals.47. The process of claim 25, further comprising the step of gravitypurification of said impurity stream.
 48. The process of claim 47,wherein said impurity stream has a final density of at least about 20%solids.
 49. A method for reducing emission of a pollutant duringcalcining of a saline mineral comprising the steps of: (a) heating saidsaline mineral in a calcining vessel to calcine said saline mineral,wherein said step of heating said saline mineral produces calcining gascomprising water vapor and a pollutant; (b) removing said calcining gasfrom said calcining vessel through a calcining gas outlet; and (c)condensing at least a portion of said water vapor from said calcininggas, wherein at least a portion of said pollutant is removed from saidcalcining gas during said condensing step.
 50. The method of claim 49,wherein a heat source for calcining said saline mineral is not in directfluid communication with said saline mineral.
 51. The method of claim49, wherein said heating step occurs in a fluidized bed reactor.
 52. Themethod of claim 49, wherein the temperature of said heating step is lessthan about 350° C.
 53. The method of claim 49, wherein the temperatureof said heating step is from about 120° C. to about 250° C.
 54. Themethod of claim 49, wherein no portion of said saline mineral is heatedin excess of about 450° C.
 55. The method of claim 49, wherein saidheating step comprises the steps of: (i) heating a fluid; and (ii)bringing the heated fluid into thermal communication with said salinemineral.
 56. The method of claim 55, wherein said step of heating saidfluid comprises the steps of: (i) combusting an energy source to produceheat and combustion gas; (ii) transferring at least a portion of saidheat to said fluid; (iii) directing at least a portion of saidcombustion gas through said combustion gas outlet which is not in directfluid communication with said calcining vessel.
 57. A method of reducingthe amount of calcining gas exiting a calcining system wherein saidcalcining gas is produced during calcining of a saline mineral in acalcining vessel comprising the steps of: (a) removing said calcininggas from said calcining vessel through a calcining gas outlet; and (b)condensing at least a portion of said calcining gas.
 58. The method ofclaim 57, wherein said calcining step comprises heating said salinemineral in said calcining vessel above its calcining temperature with aheat source to calcine said saline mineral, wherein said heat source isnot in direct fluid communication with said saline mineral and whereinsaid step of heating said saline mineral produces said calcining gascomprising water vapor.
 59. The method of claim 58, wherein said step ofcondensing at least a portion of said calcining gas comprises the stepof condensing said water vapor by cooling said calcining gas.
 60. Aprocess for producing sodium carbonate from a feedstream containingtrona and insoluble impurities, comprising the steps of: (a) heatingsaid feedstream in an inert atmosphere to form anhydrous sodiumcarbonate; (b) contacting said anhydrous sodium carbonate with asaturated sodium carbonate brine solution to form sodium carbonatemonohydrate crystals; and (c) separating at least a portion of saidsodium carbonate monohydrate crystals from at least a portion of saidinsoluble impurities.
 61. The process of claim 60, wherein saidtemperature of heating is less than about 350° C.
 62. The process ofclaim 60, wherein said temperature of heating is from about 120° C. toabout 200° C.
 63. The process of claim 60, wherein no portion of saidfeedstream is heated in excess of about 450° C.
 64. The process of claim60, wherein said heating step is conducted in a fluidized bed reactor.65. The process of claim 60, wherein said step of heating produces acalcining gas comprising carbon dioxide and water vapor.
 66. The processof claim 65, further comprising the step of condensing water vapor fromsaid calcining gas.
 67. The process of claim 66, wherein said inertatmosphere comprises said carbon dioxide.
 68. A method for thecalcination and purification of trona, comprising: (a) introducing afeedstream of trona and impurity particles to a fluidized bed reactor atan elevated temperature to calcine said trona to anhydrous sodiumcarbonate; (b) removing a bottom stream of particles from said fluidizedbed reactor; (c) removing a top stream of particles from said fluidizedbed reactor; wherein the average density of particles in said top streamis less than the average density of particles in said bottom stream, andwherein the concentration of sodium carbonate in said top stream isgreater than in said bottom stream.